1. Field of the Invention
The invention relates to novel vaccines for protecting chickens against infection with Marek's disease virus.
2. Description of the Prior Art
Marek's disease (MD), a highly prevalent and important lymphoproliferative disease of chickens, is controlled in commercial chickens by live virus vaccines consisting of attenuated or naturally avirulent MD-related herpesviruses. Although vaccination programs have been considered highly effective overall, the poultry industry continues to experience losses due to MD. Given the tendency of MD virus to become more virulent with time coupled with the economic pressures confronting the poultry industry, there is still a strong incentive to develop even more efficacious products that will protect better in the face of early challenge with very virulent field strains without causing adverse side effects. This invention relates to a novel vaccine against MD which does in fact provide superior protection and improved safety compared to certain existing commercial vaccines.
There are three distinct serotypes of MD virus found in chickens: (1) serotype 1, the oncogenic form responsible for the disease, including high- and low-virulence MD virus and their attenuated variants; (2) serotype 2, a nononcogenic MD virus; and (3) serotype 3, herpesvirus of turkeys (HVT).
The prototype MD vaccine consists of the serotype 3 virus originally isolated from turkeys as reported in Witter et al. [Am. J. Vet. Res. 31: 525-538 (1970)] and Okazaki et al. [U.S. Pat. No. 3,642,574]. Its lack of oncogenicity, self-limiting infection, good replication in vivo and in vitro, availability as cell-free and cell-associated preparations, and high protective efficacy have established HVT as a standard for MD vaccines throughout the world. A commonly used strain of HVT is FC126.
Vaccines produced from the naturally avirulent SB-1 strain [Schat et al., J. Natl. Cancer Inst. 60: 1075-1082 (1978) and U.S. Pat. No. 4,160,024], an isolate of a serotype 2 MD virus, have been licensed in the United States since 1984. The SB-1 strain is poorly protective against the highly virulent MDV strains. It is usually used in combination with HVT as a bivalent vaccine since the two viruses together produce greater protection than does either one alone [Schat et al., Avian Pathol. 11: 593-606 (1982); Witter, Avian Pathol. 11: 49-62 (1982), herein incorporated by reference]. This phenomenon has been termed “protective synergism”. The SB-1+HVT bivalent vaccine represents about 18% of the United States market for MD vaccines at present and is considered to be among the most efficacious of the various MD products available. However, sporadic losses occur despite its use.
Another MD vaccine produced from strain CVI988 clone C (CVI988/C) has been licensed for commercial use in the United States. This vaccine was derived from a mildly virulent serotype 1 MD virus attenuated by serial passage in tissue culture and has been reported by deBoer et al. [Avian Dis. 30: 276-283 (1986)]. A further passaged derivative of CVI988/C, identified as CVI988/C/R6, has also been described by de Boer et al. [Advances in Marek's Disease Research, pp. 405-413 (1988)]. More recently, the original low-passage strain, designated CVI988/Rispens, which has been in commercial use in other countries for a number of years, was found to be highly effective against challenge with several very virulent MD virus strains by Witter et al. [4th Int'l Symp. Marek's Disease, pp. 315-319 (1992)].
An experimental vaccine derived from Md11, a very virulent serotype 1 MD field isolate, was reported by Witter, supra. Md11 was attenuated by 75 serial passages in cell culture, and the resultant vaccine designated Md11/75C. This vaccine has been shown to provide good protection against challenge with Md5 and most other highly virulent MD viruses tested; but it was less efficacious against challenge with the JM/102W strain, a prototype MD virus effectively protected against by HVT and SB-1 vaccines. Furthermore, its efficacy was consistently lower in chicks with HVT antibody.
In U.S. Pat. No. 4,895,717, Witter disclosed a revertant derivative of Md11/75C which was referred to as Md11/75C/R2. Md11/75C/R2 was shown to be superior to several other monovalent vaccines and was the equal of a bivalent (HVT+SB-1) vaccine [Witter, Avian Dis. 31: 752-765 (1987)]. However, the inherent pathogenicity of serotype 1 viruses and the potential of attenuated strains to revert to greater pathogenicity (Witter et al., Avian Pathol. 13: 75-92 (1984)] are factors to be considered in the licensing of such products. A clone derived from further passages of the Md11/75C/R2 strain, designated Md11/75C/R2/23 (or R2/23), was found by Witter et al. [Avian Dis., 35:877-891 (1991)] to possess the highly protective nature of the parent strain without its residual pathogenicity.
Witter also described another MD vaccine derived from 301B/1, a nonpathogenic serotype 2 field isolate, in U.S. Pat. No. 4,895,718, the contents of which are incorporated by reference herein. Strain 301B/1 possessed superior replicative ability to SB-1, as well as greater protectivity against challenge to viruses.
Still other concerns have arisen over the use of some MD vaccines. As indicated, bivalent vaccines composed of MD virus serotypes 2 and 3 are currently widely used in the U.S. and have provided excellent protection against certain MD strains. However, use of such vaccines containing serotype 2 MD virus may lead to increased mortality from another disease, lymphoid leukosis. This enhancement of lymphoid leukosis in avian leukosis virus infected chickens resulting from vaccination with products containing serotype 2 MD virus has been an unfortunate deterrent to their expanded use.
Thus, although HVT, SB-1, CVI988, CVI988/C, Md11/75C, Md11/75C/R2 and 301B/1 are all effective against certain MD viruses, none of these vaccines protect optimally against all MD challenge viruses in all chickens. Moreover, these vaccines have exhibited reduced efficacy against some of the more recently isolated very virulent strains of MD virus. To avert any large-scale outbreaks of MD in the future, the need exists to develop improved vaccines effective against the very virulent strains of MD virus.